class Solution(object):
    def _numTrees(self, n):
        """
        :type n: int
        :rtype: int
        """
        dp = [0] * (n + 1)
        dp[0] = dp[1] = 1
        for i in range(2, n + 1):
            for j in range(1, i + 1):
                dp[i] += dp[j - 1] * dp[i - j]
        return dp[-1]
        
    def numTrees(self, n):
        ans = 1
        for i in range(1, n + 1):
            ans = ans * (n + i) / i
        return ans / (n + 1)
            
            
            
        